Home made crystal ladder filters
 

When it comes to crystal filters, it appears quickly that you have to pay a high price if you want good performance devices : so  I designed them by myself using methods proposed by W. Hayward W7ZOI,  J. Makhinson N6NWP and others.
I built 3 types of ladder crystal filters : one for SSB (2300 Hz), the other for CW (400 Hz) and the third as a noise filter. As all the modules of my home-made transceiver have 50 ohms input and output impedances, the filters include matching transformers.

First I bought a batch of 30 x 8 MHz crystals ( # $1 each ) and sorted them out by frequencies and Q using a simple test fixture. I selected 12 crystals to design an SSB filter : their main characteristics are summarized below :

Fm = 7996.582 kHz      Qm = 102000     Lm = 19.5 mH

I then selected a 2.3 kHz Chebycheff  0.1 dB design and, after many simulations with the W7ZOI programs, was ready to build the prototype : its schematic is presented below.


 

SSB Filter Frequency Response.
Some measured characteristics : BW(-3dB) = 2300 Hz, BandPass Ripple< 1 dB, Insertion Loss 6 dB, Ultimate Rejection # 100dB, 6/80dB Shape factor # 1.6
 


This picture shows the inside of the SSB ladder filter : its box is fairly large because first tests show that the same circuit, housed in a smaller box, has a much lower ultimate rejection.
 

Using the same method, I then designed and build a 400 Hz Butterworth Filter and a 2700 Hz noise filter ( schematics below) with some of the remaining crystals :

 Measured characteristics : BW(-3dB) = 360 Hz,  Ultimate Rejection < 90dB, 6/60 dB Shape factor # 3.2

  6 Poles CW Filter.
 
 

Measured characteristics : BW(-3dB) = 2700 Hz, Ultimate Rejection < 60dB, 6/40 dB Shape factor # 2.6
 
 

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